I'm not an angel anymore. I'm your new God. A better one. So you will bow down and profess your love unto me, your Lord. Or I shall destroy you.
– Castiel, 6.22 The Man Who Knew Too Much
When Castiel absorbs the souls of Purgatory, he proclaims himself a new god. He is able to obliterate the archangel Raphael with a snap of his fingers. Sam stabs him in the back with an angel blade, but as a result of absorbing the "millions upon millions of souls", he is left invulnerable to the blade's effects.
Godstiel uses his powers to kill thousands of angels who had sided with Raphael. He also kills people he deems to be immoral, and acting against his will, including hypocritical clergy and motivational speakers. At the same time, he also performs healing miracles. While he calls himself a new god, Death describes his appearance as that of a "mutated angel." As the leviathans start to take over within him, and his killing becomes indiscriminate, Castiel returns to Sam, Dean, and Bobby and they help him return the souls to Purgatory.